Seamless steel pipe for oil wells excellent in sulfide stress cracking resistance and method for producing the same

ABSTRACT

A high-strength seamless steel pipe for oil wells excellent in sulfide stress cracking resistance which comprises, on the percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05% and B: 0.0003 to 0.005%, with a value of equation “C+(Mn/6)+(Cr/5)+(Mo/3)” of 0.43 or more, with the balance being Fe and impurities, and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less. The seamless steel pipe may contain a specified amount of one or more element(s) of V and Nb, and/or a specified amount of one or more element(s) of Ca, Mg and REM. The seamless steel pipe can be produced at a low cost by adapting an in-line tube making and heat treatment process having a high production efficiency since a reheating treatment for refinement of grains is not required.

This application is a continuation of the international applicationPCT/JP2005/001186 filed on Jan. 28, 2005, the entire content of which isherein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a high strength seamless steel pipewhich is excellent in sulfide stress cracking resistance and a methodfor producing the same. More specifically, the present invention relatesto a seamless steel pipe for oil wells having a high yield ratio andalso excellent sulfide stress cracking resistance, which is produced bythe method of quenching and tempering for a specified component-basedsteel.

“An oil well” in the present specification includes “a gas well”, andso, the meaning of “for oil wells” is “for oil and/or gas wells”.

BACKGROUND ART

A seamless steel pipe, which is more reliable than a welded pipe, isfrequently used in a sever oil well environment or high-temperatureenvironment, and the enhancement of strength, improvement in toughnessand improvement in sour resistance are therefore consistently required.Particularly, in oil wells to be developed in future, the enhancement instrength of the steel pipe is needed more than ever before because ahigh-depth well will become the mainstream, and a seamless steel pipefor oil wells also having stress corrosion cracking resistance isincreasingly required because the pipe is used in a severe corrosiveenvironment.

The hardness, namely the dislocation density, of steel product is raisedas the strength is enhanced, and the amount of hydrogen to be penetratedinto the steel product increases to make the steel product fragile tostress because of the high dislocation density. Accordingly, the sulfidestress cracking resistance is generally deteriorated against theenhancement in strength of the steel product used in a hydrogensulfide-rich environment. Particularly, when a member having a desiredyield strength is produced by use of a steel product with a low ratio of“yield strength/tensile strength” (hereinafter referred to as yieldratio), the tensile strength and hardness are apt to increase, and thesulfide stress cracking resistance is remarkably deteriorated.Therefore, when the strength of the steel product is raised, it isimportant to increase the yield ratio for keeping the hardness low.

Although it is preferable to make the steel product into a uniformtempered martensitic microstructure for increasing the yield ratio ofthe steel, that alone is insufficient. As a method for further enhancingthe yield ratio in the tempered martensitic microstructure, refinementof prior-austenite grains is given. However, the refinement of austenitegrains needs quenching in an off-line heat treatment, which deterioratesthe production efficiency and increases the energy used. Therefore, thismethod is disadvantageous in these days where rationalization of cost,improvement in production efficiency and energy saving are indispensableto manufacturers.

It is described in the Patent Documents 1 and 2 that precipitation of aM₂₃C₆ type carbide in grain boundary is inhibited to improve the sulfidestress cracking resistance. An improvement in sulfide stress crackingresistance by refinement of grains is also disclosed in the PatentDocument 3. However, such measures have the difficulties as describedabove.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-73086,

Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-17389,

Patent Document 3: Japanese Laid-Open Patent Publication No. 9-111343.

DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

From the point of the above-mentioned present situation, the presentinvention has an object to provide a high strength seamless steel pipefor oil wells having a high yield ratio and an excellent sulfide stresscracking resistance, which can be produced by an efficient means capableof realizing an energy saving.

MEAN FOR SOLVING THE PROBLEMS

The gists of the present invention are a seamless steel pipe for oilwells described in the following (1), and a method for producing aseamless steel pipe for oil wells described in the following (2). Thepercentage for a component content means % based on mass in thefollowing descriptions.

(1) A seamless steel pipe for oil wells which comprises C: 0.1 to 0.20%,Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%,Al: 0.1% or less, Ti: 0.002 to 0.05%, B: 0.0003 to 0.005%, further, oneor more elements selected from one or both of the following first groupand second group as occasion demands, with a value of A determined bythe following equation (1) of 0.43 or more, with the balance being Feand impurities, and in the impurities P: 0.025% or less, S: 0.010% orless and N: 0.007% or less.

First Group:

V: 0.03 to 0.2% and Nb: 0.002 to 0.04%,

Second Group:

Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.005% and REM: 0.0003 to 0.005%,A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1),wherein, in the equation (1), C, Mn, Cr and Mo each represent % by massof the respective elements.

In order to improve the sulfide stress cracking resistance of the steelpipe for oil wells described in (1), preferably the tensile strength isnot more than 931 MPa (135 ksi).

(2) A method for producing a seamless steel pipe for oil wells, whichcomprises the steps of making a pipe by hot-piercing a steel billethaving a chemical composition described in the above (1) and a value ofA determined by the above equation (1) of 0.43 or more followed byelongating and rolling, and finally rolling at a final rollingtemperature adjusted to 800 to 1100 degrees centigrade, assistantlyheating the resulting steel pipe in a temperature range from the Ar₃transformation point to 1000 degrees centigrade in-line, and thenquenching it from a temperature of the Ar₃ transformation point orhigher followed by tempering at a temperature lower than the Ac₁transformation point.

In order to obtain more uniform microstructure, in the method forproducing a seamless steel pipe for oil well described in (2),preferably the temperature of the assist heating of the steel pipein-line is between the AC₃ transformation point and 1000 degreescentigrade.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graphic representation of the influence of the content of Con the relationship between yield strength (YS) and yield ratio (YR) ina quenched and tempered steel plate.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention has been accomplished on the basis of thefollowing findings.

The yield ratio of a steel product having a quenched and temperedmicrostructure is most significantly influenced by the content of C. Theyield ratio generally increases when the C content is reduced. However,even if the C content is simply reduced, a uniform quenchedmicrostructure cannot be obtained since the hardenability isdeteriorated, and the yield ratio cannot be sufficiently raised.Therefore, it is important for the hardenability deteriorated byreducing the C content to be improved by adding Mn, Cr and Mo.

When the A-value of the above-mentioned equation (1) is set to 0.43 ormore, a uniform quenched microstructure can be obtained in a generalsteel pipe quenching facility. The present inventors confirmed that whenthe A-value of the equation (1) is 0.43 or more, the hardness in aposition 10 mm from a quenched end (hereinafter referred to as “Jominyend”) in a Jominy test exceeds the hardness corresponding to amartensite ratio of 90% and satisfactory hardenability can be ensured.The A-value is preferably set to 0.45 or more, and more preferably 0.47or more.

The present inventors further examined the influence of alloy elementson the yield ratio and sulfide stress cracking resistance of a steelproduct having a quenched and tempered microstructure. The examinationresults are as follows:

Each of steels having chemical components shown in Table 1 was melted byuse of a 150 kg vacuum melting furnace. The obtained steel ingot was hotforged to form a block with 50 mm thickness, 80 mm width and 160 mmlength. A Jominy test piece was taken from the remaining ingotaustenitized at 1100 degrees centigrade, and submitted to a Jominy testto examine the hardenability of each steel. The prior-austenite grainsize of each steel A to G of Table 1 was about No. 5 and relativelycoarse.

The Rockwell C hardness in the position 10 mm from the Jominy end in theJominy test (JHRC₁₀) of each steel A to G and the Rockwell C hardnesspredicted value at 90%-martensite ratio corresponding to the C contentof each steel A to G are shown in Table 1. The position 10 mm from theJominy end in the Jominy test corresponds to a cooling rate of 20degrees centigrade/second. The predicted value of the Rockwell Chardness at 90%-martensite ratio based on the content C is given by“58C%+27” as shown in the following Non-Patent Document 1.

Non-Patent Document 1: “Relationship between hardenability andpercentage martensite in some low alloy steels” by J. M. Hodge and M. A.Orehoski, Trans. AIME, 167, 1946, pp. 627-642.

[Table 1] TABLE 1 Chemical composition (mass %) The balance: Fe andimpurities A- Ac₁ Ac₃ 58 C. Steel C Si Mn P S Cr Mo V Ti B Ca sol. Al Nvalue point point JHRC₁₀ % + 27 A 0.10 0.21 0.61 0.012 0.002 0.70 0.300.05 0.019 0.0010 0.0025 0.042 0.0040 0.442 758 897 35.4 32.8 B 0.150.18 0.59 0.010 0.002 0.58 0.29 0.05 0.019 0.0010 0.0025 0.042 0.00400.461 754 872 38.5 35.7 C 0.20 0.18 0.60 0.011 0.001 0.61 0.30 0.050.025 0.0012 0.0028 0.043 0.0041 0.522 753 848 41.0 38.6 D 0.27 0.180.58 0.010 0.002 0.59 0.30 0.05 0.010 0.0015 0.0025 0.033 0.0037 0.585752 816 45.8 42.7 E 0.35 0.19 0.60 0.011 0.002 0.60 0.30 0.05 0.0160.0013 0.0032 0.035 0.0048 0.670 750 778 52.5 47.3 F 0.16 0.18 0.950.010 0.002 0.30 0.12 0.05 0.015 0.0010 0.0025 0.042 0.0040 0.418 739855 34.1 36.3 G 0.20 0.38 0.79 0.011 0.001 0.59 0.68 0.05 0.008 — 0.00280.031 0.0041 0.676 765 870 36.5 38.6A = C + (Mn/6) + (Cr/5) + (Mo/3).In the columns both “Ac₁ point” and “Ac₃ point”, the temperature unit is“degrees centigrade”.JHRC₁₀ means the Rockwell C hardness in the position 10 mm from thequenched end in the Jominy test.

In the steels A to E with A-values of 0.43 or more of the said equation(1), JHRC₁₀ exceeds the Rockwell C hardness corresponding to90%-martensite ratio, and satisfactory hardenability can be ensured.

On the other hand, the steel F with an A-value smaller than 0.43 of theequation (1) and the steel G containing no B (boron) are short ofhardenability, since JHRC₁₀ is below the Rockwell C hardnesscorresponding to the 90%-martensite ratio.

Next, above-mentioned each block was subjected to a heating treatment ofsoaking at 1250 degrees centigrade for 2 hours, immediately carried to ahot rolling machine, and hot-rolled to a thickness of 16 mm at a finishrolling temperature of 950 degrees centigrade or higher. Each hot-rolledmaterial was then carried to a heating furnace before the surfacetemperature becomes lower than the Ar₃ transformation point, allowed tostand therein at 950 degrees centigrade for 10 minutes, and theninserted and water-quenched in an agitating water tank.

Each water-quenched plate was divided to a proper length, and atempering treatment of soaking for 30 minutes was carried out at varioustemperatures to obtain quenched and tempered plates. Round bar tensiletest pieces were cut off from the longitudinal direction of thethus-obtained hot-rolled and heat-treated plates, and a tensile test wascarried out.

FIG. 1 is a graphic representation of the relationship between yieldstrength (YS) and yield ratio (YR, the unit is represented by %) ofplates changed in strength by variously changing the temperingtemperature of the steels A to E. The unit of YS is represented by ksi,wherein 1 MPa=0.145 ksi. The concrete data of tempering temperature andtensile properties are shown in Table 2.

[Table 2] TABLE 2 Tensile Properties Steel Mark Tempering Temperature YS(ksi) TS (ksi) YR (%) A 1 640 118 123 96.1 2 660 112 117 95.8 3 680 107112 95.4 4 700 102 107 94.5 5 720 92 99 92.4 B 1 640 124 131 94.9 2 660119 126 94.6 3 680 112 119 94.1 4 700 98 107 92.0 5 720 85 96 88.9 C 1640 135 144 93.5 2 660 127 136 93.1 3 680 120 129 92.8 4 700 109 11991.4 5 720 97 109 89.2 D 1 640 131 143 91.4 2 660 120 132 91.2 3 680 113125 90.3 4 700 103 117 88.6 5 720 93 108 86.8 E 1 640 136 149 90.9 2 660126 140 89.7 3 680 115 129 88.9 4 700 102 118 86.6 5 720 90 106 84.8 F 1640 120 137 88.0 2 660 114 131 87.0 3 680 104 125 85.8 4 700 92 115 84.35 720 81 104 81.0 G 1 640 130 137 88.0 2 660 122 131 87.2 3 680 114 12585.4 4 700 95 105 82.0 5 720 87 104 78.0In the columns “Tempering Temperature”, the temperature unit is “degreescentigrade”.

As is apparent from FIG. 1 and Table 2, in spite of the prior-austenitegrain sizes are about No. 5, which are relatively coarse, the steels Ato C with 0.20% or less of C have yield ratios larger than the steels Dto E with 0.25% or more of C by 2% or more. Thus, this clearly showsthat a material with high yield ratio can be obtained over a widestrength range by reducing the C content in a quenched and temperedsteel while ensuring the hardenability to make the steel into a uniformquenched microstructure. It is apparent that the effect of raising theyield ratio cannot be obtained in the steels F to G even with 0.20% orless of C but insufficient hardenability.

The reason for specifying the chemical composition of the steel of aseamless steel pipe for oil wells in the present invention will be nowdescribed in detail.

C:

C is an element effective for inexpensively enhancing the strength ofsteel. However, with the C content of less than 0.1%, a low-temperaturetempering must be performed to obtain a desired strength, which causes adeterioration in sulfide stress cracking resistance, or the necessity ofaddition of a large amount of expensive elements to ensure thehardenability. With the C content exceeding 0.20%, the yield ratio isreduced, and when a desired yield strength is obtained, a rise ofhardness is caused to deteriorate the sulfide stress crackingresistance. Accordingly, the C content is set to 0.1 to 0.20%. Thepreferable range of the C content is 0.12 to 0.18%, and the morepreferable range is 0.14 to 0.18%.

Si:

Si is an element, which enhances the hardenability of steel to improvethe strength in addition to deoxidation effect, and a content of 0.05%or more is required. However, when the Si content exceeds 1.0%, thesulfide stress cracking resistance is deteriorated. Accordingly, theproper content of Si is 0.05 to 1.0%. The preferable range of the Sicontent is 0.1 to 0.6%.

Mn:

Mn is an element, which enhances the hardenability of steel to improvethe strength in addition to deoxidation effect, and a content of 0.05%or more is required. However, when the Mn content exceeds 1.0%, thesulfide stress cracking resistance is deteriorated. Accordingly, thecontent of Mn is set to 0.05 to 1.0%

P:

P is an impurity of steel, which causes a deterioration in toughnessresulted from grain boundary segregation. Particularly when the Pcontent exceeds 0.025%, the sulfide stress cracking resistance isremarkably deteriorated. Accordingly, it is necessary to control thecontent of P to 0.025% or less. The P content is preferably set to0.020% or less and, more preferably, to 0.015% or less.

S:

S is also an impurity of steel, and when the S content exceeds 0.010%,the sulfide stress cracking resistance is seriously deteriorated.Accordingly, the content of S is set to 0.010% or less. The S content ispreferably 0.005% or less.

Cr:

Cr is an element effective for enhancing the hardenability of steel, anda content of 0.05% or more is required in order to exhibit this effect.However, when the Cr content exceeds 1.5%, the sulfide stress crackingresistance is deteriorated. Therefore, the content of Cr is set to 0.05to 1.5%. The preferable range of the Cr content is 0.2 to 1.0%, and themore preferable range is 0.4 to 0.8%.

Mo:

Mo is an element effective for enhancing the hardenability of steel toensure a high strength and for enhancing the sulfide stress crackingresistance. In order to obtain these effects, it is necessary to controlthe content of Mo to 0.05% or more. However, when the Mo content exceeds1.0%, coarse carbides are formed in the prior-austenite grain boundariesto deteriorate the sulfide stress cracking resistance. Therefore, thecontent of Mo is set to 0.05 to 1.0%. The preferable range of the Mocontent is 0.1 to 0.8%.

Al:

Al is an element having a deoxidation effect and effective for enhancingthe toughness and workability of steel. However, when the content of Alexceeds 0.10%, streak flaws are remarkably caused. Accordingly, thecontent of Al is set to 0.10% or less. Although the lower limit of theAl content is not particularly set because the content may be in animpurity level, the Al content is preferably set to 0.005% or more. Thepreferable range of the Al content is 0.005 to 0.05%. The Al contentreferred herein means the content of acid-soluble Al (what we called the“sol.Al”).

B:

Although the hardenability improving effect of B can be obtained with acontent of impurity level, the B content is preferably set to 0.0003% ormore in order to obtain the effect more remarkably. However, when thecontent of B exceeds 0.005%, the toughness is deteriorated. Therefore,the content of B is set to 0.0003 to 0.005%. The preferable range of theB content is 0.0003 to 0.003%.

Ti:

Ti fixes N in steel as a nitride and makes B present in a dissolvedstate in the matrix at the time of quenching to make it exhibit thehardenability improving effect. In order to obtain such an effect of Ti,the content of Ti is preferably set to 0.002% or more. However, when thecontent of Ti is 0.05% or more, it is present as a coarse nitride,resulting in the deterioration of the sulfide stress crackingresistance. Accordingly, the content of Ti is set to 0.002 to 0.05%. Thepreferable range of Ti content is 0.005 to 0.025%.

N:

N is unavoidably present in steel, and binds to Al, Ti or Nb to form anitride. The presence of a large amount of N not only leads to thecoarsening of AlN or TiN but also remarkably deteriorates thehardenability by also forming a nitride with B. Accordingly, the contentof N as an impurity element is set to 0.007% or less. The preferablerange of N is less than 0.005%.

Limitation of the A-value Calculated by the Equation (1):

The A-value is defined by the following equation (1) as described above,wherein C, Mn, Cr, and Mo in the equation (1) mean the percentage of themass of the respective elements.A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1).

The present invention is intended to raise the yield ratio by limiting Cto improve the sulfide stress cracking resistance. Accordingly, if thecontents of Mn, Cr, and Mo are not adjusted according to the adjustmentof the C content, the hardenability is impaired to rather deterioratethe sulfide stress cracking resistance. Therefore, in order to ensurethe hardenability, the contents of C, Mn, Cr and Mo must be set so thatthe said A-value of the equation (1) is 0.43 or more. The said A-valueis preferably set to 0.45 or more, and more preferably to 0.47 or more.

The optional components of the first group and the second group whichare included as occasion demands will be then described.

The first group consists of V and Nb. V precipitates as a fine carbideat the time of tempering, and so it has an effect to enhance thestrength. Although such effect is exhibited by including 0.03% or moreof V, the toughness is deteriorated with the content exceeding 0.2%.Accordingly, the content of added V is preferably set to 0.03 to 0.2%.The more preferable range of the V content is 0.05 to 0.15%.

Nb forms a carbonitride in a high temperature range to prevent thecoarsening of grains to effectively improve the sulfide stress crackingresistance. When the content of Nb is 0.002% or more, this effect can beexhibited. However, when the content of Nb exceeds 0.04%, thecarbonitride is excessively coarsened to rather deteriorate the sulfidestress cracking resistance. Accordingly, the content of added Nb ispreferably set to 0.002 to 0.04%. The more preferable range of the Nbcontent is 0002 to 0.02%.

The second group consists of Ca, Mg and REM. These elements are notnecessarily added. However, since they react with S in steel when added,to form sulfides to thereby improve the form of an inclusion, thesulfide stress cracking resistance of the steel can be improved as aneffect. This effect can be obtained, when one or two or more selectedfrom the group of Ca, Mg and REM (rare earth elements, namely Ce, Ra, Yand so on) is added. When the content of each element is less than0.0003%, the effect cannot be obtained. When the content of everyelement exceeds 0.005%, the amount of inclusions in steel is increased,and the cleanliness of the steel is deteriorated to reduce the sulfidestress cracking resistance. Accordingly, the content of added eachelement is preferably set to 0.0003 to 0.005%. In the present invention,the content of REM means the sum of the contents of rare earth elements.

Previously described, in general, the higher the strength of a steelbecomes, the worse the sulfide stress cracking resistance becomes in thecircumstance containing much hydrogen sulfide. But the seamless steelpipe for oil wells comprising the chemical compositions described aboveretains the good sulfide stress cracking resistance if the tensilestrength is not more than 931 MPa. Therefore the tensile strength of theseamless steel pipe for oil well is preferably not more than 931 MPa(135 ksi). More preferably the upper limit of the tensile strength is897 MPa (130 ksi).

Next, the method for producing a seamless steel pipe for oil wells ofthe present invention will be described.

The seamless steel pipe for oil wells of the present invention isexcellent in sulfide stress cracking resistance with a high yield ratioeven if it has a relatively coarse microstructure such that themicrostructure is mainly composed of tempered martensite with anprior-austenite grain of No. 7 or less by a grain size number regulatedin JIS G 0551 (1998). Accordingly, when a steel ingot having theabove-mentioned chemical composition is used as a material, the freedomof selection for the method for producing a steel pipe can be increased.

For example, the said seamless steel pipe can be produced by supplying asteel pipe formed by piercing and elongating by the Mannesmann-mandrelmill tube-making method to a heat treatment facility provided in thelatter stage of a finish rolling machine while keeping it at atemperature of the Ar₃ transformation point or higher to quench itfollowed by tempering at 600 to 750 degrees centigrade. Even if anenergy-saving type in-line tube making and heat treatment process suchas the above-mentioned process is selected, a steel pipe with a highyield ratio can be produced, and a seamless steel pipe for oil wellshaving a desired high strength and high sulfide stress crackingresistance can be obtained.

The said seamless steel pipe can be also produced by cooling ahot-finish formed steel pipe once down to room temperature, reheating itin a quenching furnace to soak in a temperature range of 900 to 1000degrees centigrade followed by quenching in water, and then tempering at600 to 750 degrees centigrade. If an off-line tube making and heattreatment process such as the above-mentioned process is selected, asteel pipe having a higher yield ratio can be produced by the refinementeffect of prior-austenite grain, and a seamless steel pipe for oil wellswith higher strength and higher sulfide stress cracking resistance canbe obtained.

However, the production method described below is most desirable. Thereason is that since the pipe is held at a high temperature from thetube-making to the quenching, an element such as V or Mo can be easilykept in a dissolved state in the matrix, and such elements precipitatesin a high-temperature tempering which is advantageous for improving thesulfide stress cracking resistance, and contribute to the increase instrength of the steel pipe.

The method for producing a seamless steel pipe for oil wells of thepresent invention is characterized in the final rolling temperature ofelongating and rolling, and the heat treatment after the end of rolling.Each will be described below.

(1) Final Rolling Temperature of Elongating and Rolling

This temperature is set to 800 to 1100 degrees centigrade. At atemperature lower than 800 degrees centigrade, the deformationresistance of the steel pipe is excessively increased to cause a problemof tool abrasion. At a temperature higher than 1100 degrees centigrade,the grains are excessively coarsened to deteriorate the sulfide stresscracking resistance. The piercing process before the elongating androlling may be carried out by a general method, such as Mannesmannpiercing method.

(2) Assistant Heating Treatment

The elongated and rolled steel pipe is charged in line, namely in aassistant heating furnace provided within a series of steel pipeproduction lines, and assistantly heated in a temperature range from theAr₃ transformation point to 1000 degrees centigrade. The purpose of theassistant heating is to eliminate the dispersion in the longitudinaltemperature of the steel pipe to make the microstructure uniform.

When the temperature of the assistant heating is lower than the Ar₃transformation point, a ferrite starts to generate, and the uniformquenched microstructure cannot be obtained. When it is higher than 1000degrees centigrade, the grain growth is promoted to cause thedeterioration of the sulfide stress cracking resistance by graincoarsening. The time of the assistant heating is set to a time necessaryfor making the temperature of the whole thickness of the pipe to auniform temperature, that is about 5 to 10 minutes. Although, theassistant heating process may be omitted when the final rollingtemperature of elongating and rolling is within a temperature range fromthe Ar₃ transformation point to 1000 degrees centigrade, the assistantheating is desirably carried out in order to minimize the longitudinaland thickness-directional dispersion in temperature of the pipe.

The more uniform microstructure is obtained when the temperature of theassist heating of a steel pipe in-line is between the Ac₃ transformationpoint and 1000 degrees centigrade. Therefore, the temperature of theassist heating of a steel pipe in-line is preferably between the Ac₃transformation point and 1000 degrees centigrade.

(3) Quenching and Tempering

The steel pipe laid in a temperature range from the Ar₃ transformationpoint to 1000 degrees centigrade through the above processes isquenched. The quenching is carried out at a cooling rate sufficient formaking the whole thickness of the pipe into a martensiticmicrostructure. Water cooling can be generally adapted. The tempering iscarried out at a temperature lower than the Ac₁ transformation point,desirably, at 600 to 700 degrees centigrade. The tempering time may beabout 20 to 60 minutes although it depends on the thickness of the pipe.

According to the above processes, a seamless steel pipe for oil wellswith excellent properties formed of tempered martensite can be obtained.

PREFERRED EMBODIMENT

The present invention will be described in more detail in reference topreferred embodiments.

EXAMPLE 1

Billets with an outer diameter of 225 mm formed of 28 kinds of steelsshown in Table 3 were produced. These billets were heated to 1250degrees centigrade, and formed into seamless steel pipes with 244.5 mmouter diameter and 13.8 mm thickness by the Mannesmann-mandreltube-making method.

[Table 3] TABLE 3 Chemical composition (mass %) The balance: Fe andimpurities Steel C Si Mn P S Cr Mo B sol. Al N 1 0.12 0.26 0.91 0.0100.002 0.43 0.35 0.0012 0.024 0.0039 2 0.11 0.33 0.61 0.010 0.004 0.610.51 0.0021 0.026 0.0038 3 0.15 0.22 0.61 0.010 0.004 0.30 0.50 0.00120.025 0.0041 4 0.20 0.25 0.60 0.010 0.004 0.31 0.50 0.0013 0.029 0.00405 0.17 0.30 0.60 0.010 0.004 0.61 0.45 0.0012 0.032 0.0036 6 0.13 0.230.63 0.010 0.004 0.60 0.61 0.0003 0.031 0.0018 7 0.13 0.40 0.75 0.0110.004 0.36 0.58 0.0012 0.028 0.0037 8 0.16 0.30 0.80 0.011 0.004 0.300.51 0.0011 0.028 0.0043 9 0.15 0.19 0.82 0.010 0.004 0.25 0.40 0.00100.030 0.0047 10 0.15 0.63 0.40 0.010 0.004 0.60 0.30 0.0015 0.029 0.004111 0.16 0.19 0.62 0.010 0.004 0.89 0.16 0.0019 0.031 0.0043 12 0.14 0.220.44 0.008 0.004 0.88 0.36 0.0010 0.030 0.0035 13 0.14 0.19 0.60 0.0080.004 0.61 0.48 0.0013 0.028 0.0044 14 0.16 0.22 0.63 0.009 0.004 0.300.51 0.0011 0.026 0.0024 15 0.15 0.17 0.79 0.008 0.004 0.30 0.50 0.00130.024 0.0027 16 0.15 0.17 0.99 0.009 0.004 0.61 0.31 0.0026 0.026 0.002417 0.15 0.18 0.87 0.009 0.004 0.21 0.72 0.0022 0.028 0.0040 18 0.18 0.170.50 0.008 0.004 0.51 0.72 0.0012 0.029 0.0035 19 0.16 0.18 0.81 0.0090.004 0.51 0.73 0.0012 0.030 0.0038 20 0.13 0.20 0.57 0.006 0.003 0.570.32 0.0017 0.036 0.0049 21 0.14 0.46 0.81 0.015 0.003 0.36 0.26 0.00080.031 0.0018 22 0.17 0.33 0.68 0.011 0.003 0.87 0.16 0.0019 0.033 0.002223 0.16 0.31 0.48 0.008 0.002 0.36 0.45 0.0011 0.034 0.0038 24 0.16 0.410.48 0.012 0.003 0.10 *0.01  0.0010 0.019 0.0010 25 0.14 0.22 0.81 0.0120.002 0.16 0.08 0.0011 0.031 0.0052 26 0.12 0.33 0.61 0.008 0.003 *1.63 0.77 0.0015 0.025 0.0038 27 0.17 0.28 0.56 0.011 0.003 0.92 *0.01 0.0012 0.031 0.0041 28 *0.26  0.27 0.51 0.012 0.004 0.60 0.30 0.00100.031 0.0045 Chemical composition (mass %) The balance: Fe andimpurities A- Ac₁ Ac₃ Steel Ti Nb V Ca Mg REM value point point 1 0.018— — — — — 0.474 755 888 2 0.007 — — — — — 0.504 767 907 3 0.013 — — — —— 0.478 757 883 4 0.020 — — — — — 0.529 756 861 5 0.011 — — — — — 0.542763 875 6 0.007 — — — — — 0.558 767 896 7 0.013 — — — — — 0.520 762 9038 0.013 — — — — — 0.523 756 880 9 0.014 — — — — — 0.470 750 874 10 0.016— — 0.0012 — — 0.437 768 901 11 0.008 — — 0.0031 — — 0.495 761 861 120.008 — — — 0.0010 — 0.509 769 883 13 0.013 0.006 — — — — 0.522 765 88414 0.006 — 0.18 — — — 0.495 749 879 15 0.013 0.005 — — — — 0.508 755 87716 0.003 0.008 0.05 — — — 0.540 753 864 17 0.007 0.011 0.08 — — — 0.577754 885 18 0.011 — — 0.0021 — — 0.605 766 876 19 0.014 — 0.15 0.0019 — —0.640 757 880 20 0.012 0.002 0.13 0.0020 — — 0.446 753 884 21 0.018 — —0.0010 0.0005 — 0.434 754 888 22 0.002 — — 0.0008 0.0001 0.001 0.511 762863 23 0.011 0.003 0.08 0.0010 0.0010 — 0.462 756 884 24 0.012 — — — — —*0.263  747 874 25 0.014 — — — — — *0.334  741 869 26 0.012 — — 0.0018 —— 0.804 798 908 27 0.015 — — — — — 0.451 761 857 28 0.013 0.003 0.06 — —— 0.565 756 827A = C + (Mn/6) + (Cr/5) + (Mo/3).In the columns both “Ac₁ point” and “Ac₃ point”, the temperature unit is“degrees centigrade”.The symbol “*” means that the content fails to satisfy the conditionsspecified in the invention.

Each formed seamless steel pipe was charged in a assistant heatingfurnace of a furnace temperature of 950 degrees centigrade constitutinga heat treatment facility provided in the latter stage of a finishrolling machine (namely elongating and rolling machine), allowed tostand therein to uniformly and assistantly heated for 5 minutes, andthen quenched in water.

The water-quenched seamless steel pipe was charged in a temperingfurnace, and subjected to a tempering treatment of uniformly soaking ata temperature between 650 and 720 degrees centigrade for 30 minutes, andthe strength was adjusted to about 110 ksi (758 MPa) in terms of yieldstrength to produce a product steel pipe, namely a seamless steel pipefor oil wells. The grain size of the said water-quenched steel pipe wasNo. 7 or less by the grain size number regulated in JIS G 0551 (1998) inall the steels Nos. 1 to 28.

Various test pieces were taken from the product steel pipe, and thefollowing tests were carried out to examine the properties of the steelpipe. The hardenability of each steel was also examined.

1. Hardenability

A Jominy test piece was taken from each billet before tube-makingrolling, austenitized at 1100 degrees centigrade, and subjected to aJominy test. The hardenability was evaluated by comparing the Rockwell Chardness in a position 10 mm from a Jominy end (JHRC₁₀) with the valueof 58C%+27, which is a predicted value of the Rockwell C hardnesscorresponding to 90%-martensite ratio of each steel, and determining onehaving a JHRC₁₀ higher than the value of 58C%+27 to have “excellenthardenability”, and one having a JHRC₁₀ not higher than the value of58C%+27 to have “inferior hardenability”.

2. Tensile Test

A circular tensile test piece regulated in 5CT of the API standard wascut off from the longitudinal direction of each steel pipe, and atensile test was carried out to measure the yield strength YS (ksi),tensile strength TS (ksi) and yield ratio YR (%).

3. Corrosion Test

An A-method test piece regulated in NACE TM0177-96 was cut off from thelongitudinal direction of each steel pipe, and an NACE A-method test wascarried out in the circumstance of 0.5% acetic acid and 5% sodiumchloride aqueous solution saturated with hydrogen sulfide of the partialpressure of 101325 Pa (1 atm) to measure a limit applied stress (that ismaximum stress causing no rupture in a test time of 720 hours, shown bythe ratio to the actual yield strength of each steel pipe). The sulfidestress cracking resistance was determined to be excellent when the limitapplied stress was 90% or more of YS.

The examination results are shown in Table 4. The column ofhardenability of Table 4 is shown by “excellent” or “inferior” bycomparison between JHRC₁₀ and the value of 58C%+27.

[Table 4] TABLE 4 Tensile Properties Limit Applied Steel HardenabilityYS (ksi) TS (ksi) YR (%) Stress 1 Excellent 108 113 95.6 90% YS 2Excellent 107 112 95.5 90% YS 3 Excellent 110 117 94.0 90% YS 4Excellent 109 119 91.6 90% YS 5 Excellent 109 117 93.2 90% YS 6Excellent 106 111 95.5 90% YS 7 Excellent 108 113 95.6 90% YS 8Excellent 105 113 92.9 90% YS 9 Excellent 108 115 93.9 90% YS 10Excellent 105 113 92.9 95% YS 11 Excellent 110 117 94.0 95% YS 12Excellent 107 112 95.5 95% YS 13 Excellent 105 112 93.8 90% YS 14Excellent 110 117 94.0 95% YS 15 Excellent 110 118 93.2 90% YS 16Excellent 109 117 93.2 90% YS 17 Excellent 108 116 93.1 90% YS 18Excellent 108 114 94.7 90% YS 19 Excellent 110 118 93.2 90% YS 20Excellent 109 117 93.2 90% YS 21 Excellent 106 111 95.5 90% YS 22Excellent 108 114 94.7 90% YS 23 Excellent 110 116 94.8 95% YS 24Inferior 110 124 88.7 80% YS 25 Inferior 100 121 82.6 70% YS 26Excellent 110 116 94.8 75% YS 27 Excellent 108 117 92.3 75% YS 28Excellent 110 125 88.0 80% YS

As is apparent from Table 4, the steels Nos. 1 to 23, having chemicalcompositions regulated in the present invention, have excellenthardenability, high yield ratio, and excellent sulfide stress crackingresistance.

On the other hand, all the steels Nos. 24 to 38, out of the componentrange regulated in the present invention, are inferior in sulfide stresscrack resistance.

The steel No. 24 is too short of hardenability to obtain the uniformquenched and tempered microstructure, namely the uniform temperedmartensitic microstructure, and also poor in sulfide stress crackingresistance with a low yield ratio, since the content of Mo is out of therange regulated in the present invention.

The steel No. 25 is too short of hardenability to obtain the uniformquenched and tempered microstructure, namely the uniform temperedmartensitic microstructure, and also poor in sulfide stress crackingresistance with a low yield ratio, since the conditions regulated in thepresent invention are not satisfied with an A-value of the said equation(1) lower than 0.43 although the independent contents of C, Mn, Cr andMo are within the ranges regulated in the present invention.

The steel No. 26 is excellent in hardenability and has a high yieldratio, but it is poor in sulfide stress cracking resistance since thecontent of Cr is higher than the regulation in the present invention.

The steel No. 27 is short of hardenability, and also poor in sulfidestress cracking resistance with a low yield ratio, since the content ofMo is lower than the lower limit value regulated in the presentinvention although the A-value of the said equation (1) satisfies thecondition regulated in the present invention.

The steel No. 28 is excellent in hardenability, but it is inferior insulfide stress cracking resistance with a low yield ratio, since thecontent of C is higher than the regulation of the present invention.

EXAMPLE 2

Billets with an outer diameter of 225 mm formed of 3 kinds of steelsshown in Table 5 were produced. These billets were heated to 1250degrees centigrade, and formed into seamless steel pipes with 244.5 mmouter diameter and 13.8 mm thickness by the Mannesmann-mandreltube-making method. The steels Nos. 29 to 31 in Table 5 satisfied thechemical composition defined by the present invention.

[Table 5] TABLE 5 Chemical composition (mass %) The balance: Fe andimpurities Steel C Si Mn P S Cr Mo B sol. Al N 29 0.15 0.15 0.76 0.0100.002 0.35 0.40 0.0013 0.025 0.0032 30 0.19 0.21 0.61 0.010 0.002 0.450.30 0.0009 0.021 0.0038 31 0.14 0.32 0.66 0.008 0.001 0.41 0.71 0.00120.025 0.0041 Chemical composition (mass %) The balance: Fe andimpurities A- Ac₁ Ac₃ Steel Ti Nb V Ca Mg REM value point point 29 0.016— 0.07 0.0018 — — 0.480 750 872 30 0.013 — 0.10 — 0.0008 — 0.482 752 85531 0.013 — 0.12 0.0020 — 0.0005 0.569 761 900A = C + (Mn/6) + (Cr/5) + (Mo/3).In the columns both “Ac₁ point” and “Ac₃ point”, the temperature unit is“degrees centigrade”.

Each formed seamless steel pipe was charged in a assistant heatingfurnace of a furnace temperature of 950 degrees centigrade constitutinga heat treatment facility provided in the latter stage of a finishrolling machine (namely elongating and rolling machine), allowed tostand therein to uniformly and assistantly heated for 5 minutes, andthen quenched in water.

The water-quenched seamless steel pipe was divided in two pieces andcharged in a tempering furnace, and subjected to a tempering treatmentof uniformly soaking for each piece at a temperature between 650 and 720degrees centigrade for 30 minutes, and the strength was adjusted toabout 125 ksi (862 MPa) to 135 ksi (931 MPa) in terms of tensilestrength to produce a product steel pipe, namely a seamless steel pipefor oil wells. The grain size of the said water-quenched steel pipe wasNo. 7 or less by the grain size number regulated in JIS G 0551 (1998) inall the steels Nos. 29 to 31.

Various test pieces were taken from the product steel pipe, and thefollowing tests were carried out to examine the properties of the steelpipe. The hardenability of each steel was also examined.

1. Hardenability

A Jominy test piece was taken from each billet before tube-makingrolling, austenitized at 1100 degrees centigrade, and subjected to aJominy test. The hardenability was evaluated by comparing the Rockwell Chardness in a position 10 mm from a Jominy end (JHRC₁₀) with the valueof 58C%+27, which is a predicted value of the Rockwell C hardnesscorresponding to 90%-martensite ratio of each steel, and determining onehaving a JHRC₁₀ higher than the value of 58C%+27 to have “excellenthardenability”, and one having a JHRC₁₀ not higher than the value of58C%+27 to have “inferior hardenability”.

2. Tensile Test

A circular tensile test piece regulated in 5CT of the API standard wascut off from the longitudinal direction of each steel pipe, and atensile test was carried out to measure the yield strength YS (ksi),tensile strength TS (ksi) and yield ratio YR (%).

3. Corrosion Test

An A-method test piece regulated in NACE TM0177-96 was cut off from thelongitudinal direction of each steel pipe, and an NACE A-method test wascarried out in the circumstance of 0.5% acetic acid and 5% sodiumchloride aqueous solution saturated with hydrogen sulfide of the partialpressure of 101325 Pa (1 atm) to measure a limit applied stress (that ismaximum stress causing no rupture in a test time of 720 hours, shown bythe ratio to the actual yield strength of each steel pipe). The sulfidestress cracking resistance was determined to be excellent when the limitapplied stress was 90% or more of YS.

The examination results are shown in Table 6. The column ofhardenability of Table 6 is shown by “excellent” or “inferior” bycomparison between JHRC₁₀ and the value of 58C%+27.

[Table 6] TABLE 6 Limit Tensile Properties Applied Mark SteelHardenability YS (ksi) TS (ksi) YR (%) Stress 29-1 29 Excellent 125 13294.7 90% YS 29-2 29 Excellent 120 127 94.5 95% YS 30-1 30 Excellent 125135 92.6 90% YS 30-2 30 Excellent 121 130 93.1 95% YS 31-1 31 Excellent125 130 96.2 95% YS 31-2 31 Excellent 120 125 96.0 95% YS

As is apparent from Table 6, the steels Nos. 29 to 31, having chemicalcompositions regulated in the present invention, have excellenthardenability, high yield ratio, and excellent sulfide stress crackingresistance. In particular, the marks 29-2, 30-2, 31-1 and 31-2, whosetensile strengths are not more than 130 ksi (897 MPa), have bettersulfide stress cracking resistance.

Although only some exemplary embodiments of the present invention havebeen described in detail above, those skilled in the art will readilyappreciated that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of the present invention. Accordingly, all such modificationsare intended to be included within the scope of the present invention.

INDUSTRIAL APPLICABILITY

The seamless steel pipe for oil wells of the present invention is highlystrong and excellent in sulfide stress cracking resistance because ithas a high yield ratio even with a quenched and tempered microstructure,namely a tempered martensitic microstructure, in which theprior-austenite grains are relatively coarse gains of No. 7 or less bythe grain size number regulated in JIS G 0551 (1998).

The seamless steel pipe for oil wells of the present invention can beproduced at a low cost by adapting an in-line tube making and heattreatment process having a high production efficiency since a reheatingtreatment for refinement of grains is not required.

1. A seamless steel pipe for oil wells which comprises, on the percentby mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr:0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05%and B: 0.0003 to 0.005%, with a value of A determined by the followingequation (1) of 0.43 or more, with the balance being Fe and impurities,and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007%or less:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Crand Mo each represent % by mass of the respective elements.
 2. Aseamless steel pipe for oil wells which comprises, on the percent bymass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr:0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05%,B: 0.0003 to 0.005%, and either one or both of V: 0.03 to 0.2% and Nb:0.002 to 0.04%, with a value of A determined by the following equation(1) of 0.43 or more, with the balance being Fe and impurities, and inthe impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% orless:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Crand Mo each represent % by mass of the respective elements.
 3. Aseamless steel pipe for oil wells which comprises, on the percent bymass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr:0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05%,B: 0.0003 to 0.005%, and one or more element(s) selected from a group ofCa of 0.0003 to 0.005%, Mg of 0.0003 to 0.005% and REM of 0.0003 to0.005%, with a value of A determined by the following equation (1) of0.43 or more, with the balance being Fe and impurities, and in theimpurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Cr,and Mo each represent % by mass of the respective elements.
 4. Aseamless steel pipe for oil wells which comprises, on the percent bymass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr:0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05%,B: 0.0003 to 0.005%, either one or both of V: 0.03 to 0.2% and Nb: 0.002to 0.04%, and one or more element(s) selected from a group of Ca of0.0003 to 0.005%, Mg of 0.0003 to 0.005% and REM of 0.0003 to 0.005%,with a value of A determined by the following equation (1) of 0.43 ormore, with the balance being Fe and impurities, and in the impurities P:0.025% or less, S: 0.010% or less and N: 0.007% or less:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Crand Mo each represent % by mass of the respective elements.
 5. Theseamless steel pipe for oil wells according to claim 1, wherein thetensile strength is not more than 931 MPa.
 6. The seamless steel pipefor oil wells according to claim 2, wherein the tensile strength is notmore than 931 MPa.
 7. The seamless steel pipe for oil wells according toclaim 3, wherein the tensile strength is not more than 931 MPa.
 8. Theseamless steel pipe for oil wells according to claim 4, wherein thetensile strength is not more than 931 MPa.
 9. A method for producing aseamless steel pipe for oil wells, which comprises the steps of making apipe by hot-piercing a steel billet having a chemical compositionaccording to claim 1, with a value of A determined by the followingequation (1) of 0.43 or more followed by elongating and rolling, andthen finally rolling at a final rolling temperature adjusted to 800 to1100 degrees centigrade, assistantly heating the resulting steel pipe ina temperature range from the Ar₃ transformation point to 1000 degreescentigrade in-line, and then quenching it from a temperature of the Ar₃transformation point or higher followed by tempering at a temperaturelower than the Ac₁ transformation point:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Crand Mo each represent % by mass of the respective elements.
 10. Themethod for producing a seamless steel pipe for oil wells according toclaim 9, wherein the temperature of assistant heating in-line is the Ac₃transformation point to 1000 degrees centigrade.
 11. A method forproducing a seamless steel pipe for oil wells, which comprises the stepsof making a pipe by hot-piercing a steel billet having a chemicalcomposition according to claim 2, with a value of A determined by thefollowing equation (1) of 0.43 or more followed by elongating androlling, and then finally rolling at a final rolling temperatureadjusted to 800 to 1100 degrees centigrade, assistantly heating theresulting steel pipe in a temperature range from the Ar₃ transformationpoint to 1000 degrees centigrade in-line, and then quenching it from atemperature of the Ar₃ transformation point or higher followed bytempering at a temperature lower than the Ac₁ transformation point:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Crand Mo each represent % by mass of the respective elements.
 12. A methodfor producing a seamless steel pipe for oil wells, which comprises thesteps of making a pipe by hot-piercing a steel billet having a chemicalcomposition according to claim 3, with a value of A determined by thefollowing equation (1) of 0.43 or more followed by elongating androlling, and then finally rolling at a final rolling temperatureadjusted to 800 to 1100 degrees centigrade, assistantly heating theresulting steel pipe in a temperature range from the Ar₃ transformationpoint to 1000 degrees centigrade in-line, and then quenching it from atemperature of the Ar₃ transformation point or higher followed bytempering at a temperature lower than the Ac₁ transformation point:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Crand Mo each represent % by mass of the respective elements.
 13. A methodfor producing a seamless steel pipe for oil wells, which comprises thesteps of making a pipe by hot-piercing a steel billet having a chemicalcomposition according to claim 4, with a value of A determined by thefollowing equation (1) of 0.43 or more followed by elongating androlling, and then finally rolling at a final rolling temperatureadjusted to 800 to 1100 degrees centigrade, assistantly heating theresulting steel pipe in a temperature range from the Ar₃ transformationpoint to 1000 degrees centigrade in-line, and then quenching it from atemperature of the Ar₃ transformation point or higher followed bytempering at a temperature lower than the Ac₁ transformation point:A=C+(Mn/6)+(Cr/5)+(Mo/3)  (1), wherein, in the equation (1), C, Mn, Crand Mo each represent % by mass of the respective elements.